C89 allowed a type specifier to be elided with the resulting type being
int, aka implicit int behavior. This feature was subsequently removed
in C99 without a deprecation period, so implementations continued to
support the feature. Now, as with implicit function declarations, is a
good time to reevaluate the need for this support.

This patch allows -Wimplicit-int to issue warnings in C89 mode (off by
default), defaults the warning to an error in C99 through C17, and
disables support for the feature entirely in C2x. It also removes a
warning about missing declaration specifiers that really was just an
implicit int warning in disguise and other minor related cleanups.

user interface and documentation, and update __cplusplus for C++20.

WG21 considers the C++20 standard to be finished (even though it still
has some more steps to pass through in the ISO process).

The old flag names are accepted for compatibility, as usual, and we
still have lots of references to C++2a in comments and identifiers;
those can be cleaned up separately.